Many configurations of heads for cutting the vegetation are known, the so-called mowing or grass-cutting heads, wherein the cutting member is composed of a line in synthetic material, typically nylon®, which is radially projecting from the head and which cuts the surrounding vegetation due to the effect of the fast rotation of the head controlled by the drive shaft of the brush cutter on which the head is mounted. Usually, one or more cutting lines are wound on one or more spools housed inside the head to allow restoring the length of the portions of the cutting line, which project from the head, following breakage or wear due to the use.
In some heads of this type, lengthening is carried out by pressing the head against the ground without stopping the grass cutting operations, thanks to the presence of a particular elastic mechanism provided inside the housing in which the spool is arranged, or inside the spool itself. The elastic mechanism allows the spool to move axially with respect to the housing when the head is pressed against the ground. With this alternating movement in axial direction the spool is disengaged from a series of teeth or projections provided on the bottom of the housing and engages with a corresponding series of teeth arranged on the upper part of the housing with respective projections integral with the spool itself. The teeth and/or the projections are angularly offset in such a way that the alternating movement of the spool inside the head causes an angularly stepped forward movement of the spool with respect to the housing and hence a delivery of limited quantities of line towards the outside of the head thanks to the effect of the centrifugal force which acts on the members formed by the head during the rotation.
Heads of this type are described for example in WO-A-2006/036455, WO-A-2005/110685, WO-A-2004/062347, U.S. Pat. No. 5,765,287.
In some known embodiments, the mutually cooperating teeth and/or projections are shaped so as to allow, besides the lengthening of the line, winding of the line by manual rotation of the spool with respect to the housing. At this end, a series of teeth, integral with the spool or with the housing, must be shaped with an inclined surface, which allows the reciprocal rotation between spool and housing, overcoming the elastic force of the spring which elastically stresses the spool. Angular locking between spool and housing is thus ensured only in one direction of rotation and not in the other direction. Therefore, these heads can be mounted selectively only on brush cutters, which rotate clockwise or counter-clockwise (i.e. with left or right direction of rotation) based upon the configuration of the head. The same head cannot be mounted alternatively on brush cutters with right rotation and left rotation.
This involves some drawbacks both for manufacturers and for resellers, as well as for the users of this type of garden accessories. In fact, the manufacturers must have in their catalogue for each head the double version for brush cutters with right rotation and with left rotation respectively, or they will lose potential sales volumes, i.e. relinquishing selling their accessories to the customers having a brush cutter which rotates in opposite direction to the direction for which the head has been designed. Analogously, the resellers must have both head versions in stock. In any case, for the users there is the risk of buying a head not suitable for the type of brush cutter with which he/she is provided.